by Prestressing refers to the stress applied to the structure in advance before the external load is applied to the member.
Prestress is generally opposite to the stress caused by load.
There are many examples of applying prestress in daily life.
The wooden basin (barrel) applies pre compression stress to the wood chips with hoops to offset the tensile stress generated by water.
The preload stress shall be applied by hand to offset the bending tensile stress caused by the self weight of the book.
1.
The prestressed reinforcement is used to apply the pre-stressed stress to the concrete member.
Where the prestressed concrete beam generates the tensile stress under the load, the pre-stressed reinforcement is used to apply the compressive stress to it in advance to partially or completely offset the tensile stress generated by the load.
Prestressing can improve the crack resistance of concrete members; Ø the deflection caused by the dead load is offset by the reverse arch generated by the prestress; Ø make full use of the tensile properties of high-strength steel and the compressive properties of high-strength concrete; Ø the section size can be reduced and the self weight of the structure can be reduced so as to be applied to large-span and high-rise buildings.
2.
At present, the span of this kind of pre bending steel composite simply supported beam has reached 40m.
If secondary pouring is applied to form a composite continuous beam of pre bent section steel, the advantages of this structure can be fully exerted, and the maximum span can reach 80m.
This new type of prestressed composite structure has the advantages of light weight, high bearing capacity, small deflection, small structural height, and easy construction.
It has a good application prospect in urban overpasses.
It is envisaged that a non reinforced concrete bridge will be built between the two mountains: temporary supports will be erected and prefabricated concrete blocks will be paved; Ø pre stress the precast concrete block with a row of jacks; Ø pour concrete between jacks; Ø remove the jack after reaching the necessary strength; Ø re pour concrete at the position occupied by the original jack; Ø remove the temporary support, and a prestressed bridge without prestressed reinforcement will be completed.
(this is the same as the principle of carrying books by hand.) 3.
Prestressing is an internal force.
Will prestressing a thin plate destabilize it? When the compression bar is bent due to accidental eccentricity or accidental lateral force, the additional moment may make the member more and more bent and even lead to failure, which is instability.
The thin plate or long thin rod under axial force may be unstable.
Prestressing is an internal force.
When a prestressed thin plate is accidentally bent, the additional moment generated by the prestressing force and the lateral pressure of the prestressed tendon on the concrete plate after bending are opposite to the direction of accidental bending, which will straighten the member.
It can be seen that prestressing will not only make the compression bar unstable, but also make the compression bar more stable.
It is not difficult to understand why we sometimes apply prestress to prefabricated long columns and long piles.
4.
Other applications of prestressing force the concept of prestressing force has more general significance.
The prestressing mentioned above can make the joints of the wooden basin that cannot be tensioned “tensile”, and also make the concrete with low tensile strength seem to be able to bear a large “tensile stress”.
In fact, prestressing only changes the process of tension into the process of reduction of prestressing stress.
According to the same principle, pretension stress can also be applied to make the material that cannot be pressed seem to be able to be pressed.
The balloon membrane can not be compressed.
If the membrane is subjected to tensile stress by inflating and pressing, as long as the pre tensile stress is large enough to exceed the compressive stress of the membrane under the load, the structure can bear the load.
All kinds of inflatable structures can be constructed by using this principle.
Balloon structure is an air bearing inflatable structure with low air pressure.
Take out the 1m2 roof at the top for analysis.
Generally, the self weight of the membrane material will not exceed q = 100N / m2.
As long as the internal air pressure P > Q, the membrane can be lifted.
As we all know, a standard atmospheric pressure is equivalent to about 10m water column pressure, that is, 100000n / m2.
It can be seen that as long as the indoor air pressure is about 1 / 1000 higher than the outdoor air pressure, the film can be lifted.
Such a pressure difference can be realized with only one ordinary blower.
Generally, as long as the airtight revolving door is adopted at the entrance of this air bearing inflatable structure, and the air is continuously replenished by the fan to maintain the air pressure difference of 150-300n / m2, it is enough to bear the gravity of the covering layer and maintain a certain prestress in the membrane to ensure the overall rigidity of this air bearing inflatable structure.
The indoor and outdoor pressure difference is only 1.5/1000-3 / 1000, which people cannot feel.
Trachea inflation structure, also known as pneumatic inflation structure, the air pressure in the trachea is about 1.5-2.0kn/m2.
This kind of air pipe has certain rigidity, bending resistance and compression resistance, and can be used as an arch ring to form a roof.
The inner pressure of the gas tube inflatable structure is much larger than that of the air bearing structure, but it is only about 1 / 1000 of the inner tube pressure of the automobile (about 2000kN / m2), which is easy to reach.
Trachea (bladder) inflatable structure this structure has good thermal insulation performance, but the working pressure is slightly high, and the material strength, air tightness and other quality requirements are also high, and the cost is also higher than that of the air bearing inflatable structure.
The inflatable structure is mixed with other structures, and the effect is also very good.
The inflatable structure is light in weight, low in cost and easy to assemble and disassemble.
It is especially suitable for large-span markets, exhibitions, temporary performance venues and competition venues..
